1/ The 50 states and the District of Columbia report travel by highway category, number of motor vehicles registered, and total fuel consumed. The travel and fuel data by vehicle type and stratification of trucks, as well as related data, are calculated by the Federal Highway Administration (FHWA). Entries for 1999 may have been revised based on the availability of more current data. Estimation procedures include use of the 1997 Census of Transportation Vehicle Inventory and Use Survey (VIUS) and independent analysis of light truck travel.

2/ Other 2-Axle 4-Tire Vehicles which are not passenger cars. These include vans, pickup trucks, and sport/utility vehicles.

3/ Single-Unit 2-Axle 6-Tire or More Trucks on a single frame with at least two axles and six tires.

4/ Urban consists of travel on all roads and streets in urban places with 5,000 or greater population.

5/ Stratification of the truck figures is made by the FHWA based on State-supplied data and the 1997 VIUS. Combination trucks represent approximately the number of tractors with semi-trailer(s) and a majority of heavy single-unit trucks used regularly in combination with trailer(s).

6/ As estimated by the FHWA using the 1995 Nationwide Personal Transportation Survey (NPTS).

7/ Total fuel consumption figures are derived from state fuel tax records and reflect latest available data. Distribution by vehicle type is estimated by the FHWA based on miles per gallon for both diesel and gasoline powered vehicles using State-supplied data, the 1997 VIUS, and other sources as a baseline.

The data now on the website for 2000-2006 were estimated using a methodology developed in the late 1990s. FHWA recently developed a new methodology and used it for this year's Highway Statistics. This methodology takes advantage of additional and improved information available beginning in 2007 when states were first required to report motorcycle data – before that time, the reporting was not mandatory and the data were missing for a few states. Also, the new methodology does not rely on data from the national vehicle inventory and use survey which provided critical data for the original methodology but was not collected in 2007 as planned.

In April 2011, FHWA recalculated the 2000-2008 data along with the 2009 data to estimate trends. However, after further review and consideration, the agency determined that it is more reliable to retain the original 2000-2006 estimates because the information available for those years does not fully meet the requirements of the new methodology. Thus, the original 2000-2006 estimates are now used, whereas the 2007-2009 data are still based on the new methodology.